In rimfire ammunition cartridges, the cartridge casing has a flanged rim at its closed end or base. The opposed end of the casing is open and a lead bullet is fitted therein. Primer is provided at the rim and the rest of the shell contains a propellant. The cartridge is fired when a striking pin strikes the rim of the shell, deforming the rim and igniting the primer. The primer in turn ignites the propellant and expanding gasses cause the bullet to be propelled out of the shell and down the barrel of the firearm. Because the rim of the shell is deformed during firing, rimfire cartridge casings are not reusable.
Another major type of ammunition cartridge in use today is the centerfire cartridge. Here, the primer is provided in a replaceable priming cap located in the center of the cartridge casing base. The primer is ignited when a striking pin strikes the priming cap. In centerfire cartridges, the cartridge casing can be reused by reloading the bullet, propellant and priming cap. This is an advantage for large-caliber ammunition, where the cartridge casing may be quite expensive. Because the propellant of rimfire ammunition is ignited by deformation of the rim of the cartridge casing, the rim is weakened on firing. This limits the pressure to which the cartridge casing can be subjected. For this reason, rimfire ammunition is unsuitable for most higher caliber and higher pressure ammunition, and has been all but abandoned except in .22 caliber ammunition where the economic benefit of the rimfire design outweighs its disadvantages.
The bullet of an ammunition cartridge generally has a cylindrical body portion, a flat back end at its base and a pointed nose or tip. The base of the bullet may be of a heeled or a non-heeled design.
In non-heeled bullets, the cylindrical body portion extends uniformly to the base of the bullet, so that the diameter of the bullet at its base is essentially equivalent to the diameter of the bullet at the body portion. The maximum diameter of the non-heeled bullet must therefore be smaller than the diameter of the cartridge casing at its open end, to allow the base of the bullet to fit into the cartridge casing. The advantage of non-heeled bullets is that the bullet can be lubricated with wax or grease on the base portion that is received within the cartridge casing, thereby protecting the wax or grease lubrication from contamination. A further advantage of this design is that, because the cylindrical body portion extends all the way to the base of the bullet, the bullet has a relatively long surface that bears against the barrel of the firearm, resulting in a good gas seal between the bullet and the barrel.
In heeled bullets, however, the base portion of the bullet is inwardly stepped to form a heel having a narrower diameter than the diameter of the body portion of the bullet. The heel portion of the bullet is received within the open end of the cartridge casing so that the body portion is flush with, and of the same diameter as, the cartridge casing at its open end. The entire cartridge is chambered in the barrel of a firearm, in use, and this has the advantage that ammunition cartridges of different lengths can be used in the same firearm. Heeled bullets are typically used in rimfire ammunition, such as .22 caliber ammunition. A disadvantage of heeled bullets, however, is that the exposed portion of the bullet must be lubricated with a grease or wax to avoid lead residue build up in the barrel of the firearm and this exposed lubricant is easily contaminated.
Another disadvantage of heeled bullets is that, when the bullet is expelled from the cartridge casing, the heel of the bullet is deformed and expands or “sets-up” to form part of the surface that bears against the barrel. The degree of set-up of the heel depends on the firing pressure. If the firing pressure is low, the heel will not set-up sufficiently to bear against the barrel, resulting in a potentially poor gas seal with resultant low accuracy. If the firing pressure is too high, the heel of the bullet may crack or rupture, affecting the centre of mass and bearing surface of the bullet and also resulting in poor accuracy. It is also possible that the heel may still be expanding when the bullet leaves the barrel of the firearm, with potentially uneven expanding forces resulting in destabilization of the flight trajectory of the bullet. Furthermore, hot gasses expelled by the propellant erode the base of the bullet, causing inconsistencies in the bullet that affect its balance and flight path.
Unjacketed bullets have all of the disadvantages set out above. It is known to provide a coating for a lead bullet by means of an electo-plating process. However, this coating is too thin to overcome the disadvantages set out and also varies in thickness, leading to inaccuracy.